Beginners and Beyond

I'm doing an off road tri series and even though they are short events, I like to have water with me both on the bike and on the trail. My mountain bike is so small that the water bottle cage mounts to the bottom of the down tube (stupidest idea ever) so carrying a water bottle is pretty much out of the question so I need to carry a pack. I figure if I get a small one I can just run with it too.
I have the Camelbak siren (old) http://www.amazon.com/CamelBak-Siren-Ash-Grey/dp/images/B0006I9J5S and it was fine for the bike but it bounced around on the run. Luckly, I had it pulled tight enough that there were straps hanging from the shoulder straps and I just tied them in a knot around my waist for the run and it was good to go.
SO - any suggestions for a pack idea to use for both? It doesn't have to hold much water. 16oz is fine but they probably come with 50oz bladders.
Gracias.

I am pretty sure mine is Nathan - I have been meaning to check for you.
But something like this I think will work well and not bounce (even though it's still a larger bladder): http://www.nathansports.com/hydration/race-vests/minimist

I am pretty sure mine is Nathan - I have been meaning to check for you.
But something like this I think will work well and not bounce (even though it's still a larger bladder): http://www.nathansports.com/hydration/race-vests/minimist

The pockets are handy. My phone is HUGE (Galaxy Nexus) and I can fit it in the zipper pocket, and I usually stash a gel or two in the bottle pocket. Plus there's a big pocket on the back for other stuff. Having the front pockets, in my mind, is essential for grabbing stuff without interrupting your run.